This application claims priority to German application No. 161 34 812.6, filed Jul. 17, 2001.
1. Field of the Invention
The invention relates to a flux salt composition for flux baths which is accessible for particularly simple regeneration and deferrizing.
2. Description of the Related Art
To achieve a high-quality zinc coating, the workpieces which are to be galvanized pass through a number of process steps prior to the hot-dip galvanization. After a degreasing stage to remove organic contaminants and a subsequent pickling stage for the acidic removal of oxidic contaminants, the workpieces, after rinsing, pass through a pretreatment stage in a flux bath which contains the fluxing agent solution. The purpose of this bath is to protect the workpieces from corrosion on their way to the zinc bath and during drying.
Fluxing agent solutions are aqueous salt solutions with a salt content of 300 to 500 g/l. The main constituents of these flux salts are zinc chloride and ammonium chloride. In addition, there may be lesser quantities of various alkali metal and alkaline earth metal chlorides (for example KCl, NaCl, MgCl2, CaCl2). In the case of low-fume flux salts, in particular salt mixtures, which, however, on account of the lower pickling effect are only seldom employed, the ammonium chloride content is mostly replaced by KCl.
In the case of dry galvanizing, the flux salt is applied to the material to be galvanized by dipping the workpiece into the fluxing agent solution. Even during drying there is a certain pickling effect on account of the formation of hydroxozinc acids. On dipping into the zinc pot, the dried-on flux salt is melted. For the activity of flux salts, it is important for their melting point to be well below the temperature of the zinc bath (approximately 450xc2x0 C.), so that they can rapidly effect their pickling action. The pickling action is based on the release of hydrochloric acid, which is preferably formed from ammonium chloride in the temperature range from 250 to 320xc2x0 C. This hydrochloric acid dissolves oxide contaminants.
During operation, foreign substances accumulate in the fluxing agent solution as a result of carry-over. Even when the degreasing stage is carried out carefully, it is impossible to completely prevent organic substances from being carried over into the subsequent pickling stage and onward into the flux bath. However, the iron which is carried over from the pickling baths is of relatively considerable importance. The iron accumulates in the pickling solution in the form of FeCl2, it being possible for the iron contents to be in the order of magnitude of 100 to 160 g/l. Alloying constituents of the steel grades used during pickling are also dissolved to a small extent. The introduction of iron salts, hydrochloric acid, pickling inhibitors and alloying constituents into the downstream flux bath is highly dependent on the rinsing technique employed, but cannot be avoided entirely even in the event of a high outlay on rinsing.
The pickling action of the flux salt itself plays a role as a further source of contaminants. The flux bath contains different proportions of hydrochloric acid, with the result that iron and alloying elements are dissolved out of the material to be galvanized in small quantities.
Iron which is introduced with the flux salt into the galvanizing vessel during hot-dip galvanization reacts with the elemental zinc and forms hard zinc (iron/zinc solid solutions), which precipitates as a solid in the zinc vessel. 1 g of iron forms approximately 25 g of hard zinc (Bxc3x6ohm, 1974, xe2x80x9cAbwassertechnik in Feuerverzinkereienxe2x80x9d [Wastewater technology in hot-dip galvanization plants] 12 (1974) No. 11, 235-239). The zinc losses are therefore considerable, and consequently the iron content in the flux bath should not exceed 10 g/l (Maaxcex2, Peixcex2ker xe2x80x9cFeuerverzinkenxe2x80x9d [Hot-dip galvanization] Handbook, 2nd edition, Deutscher Verlag fxc3xcr Grundstoffindustrie, Leipzig, 1993, p. 72). Hitherto, however, the flux salts have often been replaced only at iron contents of 80 to 100 g/l, and in extreme cases even only at 150 g/l. If the iron concentrations are high, the galvanization quality is impaired in addition to the zinc losses. Hard zinc crystals, which float in the zinc melt, settle on the surface of the material which is being galvanized and then appear as what are known as pimples. In addition to pimples, other flaws may also occur. For example, the presence of fine hard zinc crystals may locally increase the viscosity of the zinc melt to such an extent that when the workpieces are pulled out of the zinc pot, galvanization flaws, such as streaks and what are known as curtains, are formed. In the flux bath, pickling acid which has been carried over also results in increased dissolution of iron and therefore in an increased formation of hard zinc in the zinc pot. Old fluxes may have acid contents of more than 10 g/l and therefore a pH of less than 1.
The concentration of the carried-over organic substances from the degreasing and the pickling in the flux bath is generally low and does not have any adverse effects on quality during the galvanization. However, the organic substances are reacted in the zinc pot with the reaction partners which are present (for example zinc, chlorine, ammonium) in an uncontrolled fashion, so that pollutant-containing reaction products (for example dioxin-containing reaction products) may form, and these products, in relatively large quantities, lead to operating problems in the cleaning of the outgoing air (blocking of the filters) and make it more difficult or impossible to recycle the filter dusts which have been separated out.
Therefore, contaminated flux baths have to be exchanged regularly, the iron content which leads to replacement in hot-dip galvanization plants fluctuating within wide ranges (40 to over 80 g/l). In the past, it has only been possible to recover a small proportion of the constituents when recycling old fluxes of this type, while the majority has had to be disposed of as special waste. These methods of the prior art are generally based on using a multistage process in which, first of all, the pH is set to between 3.5 and 4, and then the divalent iron which is present in the flux bath is precipitated as iron(III) hydroxide through the addition of hydrogen peroxide, and is then separated from the fluxing agent solution by filtration in an operation which is complex and lengthy, on account of the streaky consistency of these iron hydroxide flocs. Only some of the fluxing agent solution can be reused in this method. A variant on this method, in which in each case only some of the fluxing agent solution is treated in a separate installation, is described in DE 20 29 580 C3. The fundamental drawbacks, namely the low level of reutilization and the complex method, as well as the fact that the operating staff are exposed to toxic and etching chemicals, however, are not overcome. Since the recovery of flux salt is low in the method described, many galvanization plants do not make the effort to recycle the fluxing agent solution and prefer to dispose of the entire fluxing agent solution when the iron content of the fluxing agent solution has risen to a defined maximum level. This means that considerable environmental pollution by heavy metal salts is unavoidable.
An alternative method is described in DE 38 14 372 A1. According to this method, a certain quantity is continuously or discontinuously removed from the flux bath and is rendered alkaline using a lye in a separate reactor and provided with an oxidizing agent in order to oxidize iron(II) to iron(III). This iron is bonded in an ion exchanger, preferably using hydrochloric acid, and is returned to the pickling tank after it has been separated out as iron-laden re-extraction acid. The fluxing agent solution from which iron has been removed is fed back to the flux bath after this method has been carried out. This method has the drawback of being complex and expensive and not ensuring that carried-over organic contaminants are eliminated.
A method which allows the active materials contained in the old fluxes to be almost completely recovered and the foreign substances, in particular iron and organic contaminants, to be quantitatively separated out, has been proposed in the prior art. With this method, the principle of circulation through external treatment is used to obtain, from the old fluxes, a fluxing agent solution which is as good as new and is reused in the galvanization plants. Organic contaminants are destroyed or are concentrated in a precipitation slurry, the quantitative levels and hazardous potential of which are considerably lower than with other methods which are known from the prior art. However, for process engineering reasons this method can only be used to process old flux solutions with iron contents of up to 20 g/l, which requires the flux bath to be changed more frequently, which is more than compensated for by the economic and ecological advantages of complete recycling of the flux salt, the low pollutant content of the waste sludge and the considerably improved quality of galvanization. In the method, first of all hydrogen peroxide and ammonia are added to the old flux in a number of method steps, in order to precipitate iron, and then the organic impurities are broken down by the addition of potassium permanganate. After the iron hydroxide sludge has been filtered out, manganese(II) which is included is oxidized to manganese dioxide using hydrogen peroxide and is separated out by filtration, with the result that, ultimately, a fluxing agent solution which is as good as new is obtained.
However, this method of the prior art likewise involves a number of stages and in particular is reliant on the old fluxes being reprocessed centrally, entailing high transport costs, and having to be returned to the galvanization plants in liquid form.
Therefore, in view of the increasing focus on galvanization plants with regard to environmental protection and the efforts made by the galvanizing operators to carry out the recycling of the fluxing agent solution under their own auspices, an object of the present invention is to provide a flux salt composition which allows simple recycling of the fluxing agent solution in one step and is not reliant on external, central recycling installations. As a result, the flux can be used even in markets which are not accessible for conventional recycling methods, for reasons of transport costs, for example overseas countries.
According to the invention, the above objects are achieved by a deferrizing flux salt composition for flux baths, containing zinc chloride and alkali metal chloride, which contains at least one or more alkalizing constituents and one or more constituents which, in aqueous solution, oxidize iron(II) to iron(III).